This invention relates to hermetic turbine generators, and more particularly to a Rankine cycle turbine generator system for use with low temperature heat sources.
The increasing cost and scarcity of oil has intensified the search for alternative energy sources and more efficient uses of our present energy supplies. An enormous source of energy which in a sense falls within both of these categories is waste heat from industrial processes. This heat, which is often in the range of 250.degree.-1000.degree. F., has been considered in the past to be too low in energy content to warrant the cost of reclaiming for productive use. However, with the cost and scarcity of traditional energy sources increasing at an alarming rate, we believe the economies warrant an effort to develop means for reclaiming this waste heat.
Many industrial processes extensively used in the U.S. today are highly energy intensive; that is, they consume great quanities of energy in operation of the process. Examples of such processes are chemical synthesizing and refining plants for production of urea, ammonia, plastics, rubber, and pharmaceuticals; metal smelting; refining and treating plants for aluminum, steel, coke, taconite, mercury, copper, and the many specialty metal alloys; and other processes such as petroleum refining, paper making, textiles, glass making and fabricating, power generation, ceramics, etc.
Using one of these examples to examine the energy recovery potential, consider petroleum refining. The crude petroleum is heated in a fractional distillation tower, perhaps 150 feet high. In the tower, a vertical temperature gradient is established which determines the petroleum fractions in the ascending stream of vaporized petroleum which will condense at the various levels. The streams of petroleum fractions emerging from the tower are at their condensing temperature which may range from 150.degree.-500.degree. F., at flow rates of as much as 100,000 gallons per hour, and containing heat in excess of 100 million Btu's per hour. This vast quantity of heat is presently rejected to cooling water and thence to the atmosphere via cooling towers or bodies of water, such as rivers or ocean inlets. This heat loss, if converted to electricity, could amount to more than three megawatts for each stream of petroleum fractions coming from the tower or more than 20 megawatts of electric power for a typical refinery. This power, now wasted to the detriment of the environment would greatly contribute to alleviating the current and worsening energy shortage.
The reason that this heat is rejected instead of being recovered and applied to useful work relates essentially to its low temperature. Existing electrical power generation machinery of acceptable efficiency requires heat at an input temperature of approximately 2500.degree. F. These are the only existing machines that can satisfy the industry criterion of efficiency which is represented by a discounted cash flow (DCF) figure of 25%, meaning that the per annum value of the electrical energy produced by the machine must equal at least 25% of its installed cost. The DCF figure for existing electrical generation equipment designed to utilize waste heat from industrial processes has usually been much lower than the threshold 25% figure.
The low DCF figure for waste heat recovery equipment is a direct consequence of the low temperature level of the heat source. The low source temperature forces the thermodynamic power cycle to operate at low temperatures, which results in low cycle efficiency. This is illustrated by considering the Carnot cycle, which is the theoretical, most efficient power cycle operating between a given heat source temperature and a given heat sink temperature. For a heat-rejection or sink temperature of 100.degree. F., a system with a maximum cycle temperature of 2500.degree. F. has a Carnot or maximum theoretical efficiency of 81%, whereas a system with maximum cycle temperature of 300.degree. F. has a Carnot efficiency of only 26%. The actual or real cycle efficiencies are significantly lower than these values due to the imperfections of the system components; waste heat recovery power cycles have actual efficiencies ranging from 10 to 20 percent. This means that only 10 to 20 percent of the heat energy flowing through the system equipment can be converted to useful output power. Heat exchangers are therefore disproportionately large for a given output power. This effect is further compounded by the generally low temperature differentials and pressure drops which must be maintained in process-fluid heat exchangers, which further increases heat exchanger size and cost. In addition, the energy drop of the working fluid through the prime mover of the installation is much lower than with conventional powered generators, so a much greater quantity of the low temperature fluid must pass through the machine to equal the energy output of a much smaller quantity of high temperature working fluid. As a consequence, the prime mover must be much bigger and more expensive than the prime mover for the high temperature system.
A low temperature or waste heat recovery system has one inherent advantage: the energy input is "free" in the sense that it is supplied by heat which would otherwise be wasted. This advantage is an increasingly significant one, but has remained an insufficient economic advantage to overcome the inherent disadvantages mentioned previously. Thus, to become economically attractive, a system must provide other advantages, such as low initial cost, high component efficiencies, low maintenance cost, high reliability (i.e. low downtime), small space requirements, speed and ease of installation, and durability (longevity).